Daniel Y. Jung

Theab initiohigh-pressure solid solution behaviour of the Al2O3–MgSiO3system

With the assumption that the lower mantle is pyrolitic, (Mg,Fe)SiO3 perovskite (70 vol%) is thought to be the most abundant phase in the Earths lower mantle, followed by magnesiowüstite (Mg,Fe)O with (20 vol%) and CaSiO3 perovskite, which comprises between 6 and 12 vol%. The Al2O3 content of fertile mantle compositions amounts to about 4-6 mol% and is supposed to dissolve mainly into MgSiO3 perovskite. Experimental and theoretical studies have shown that a fair amount of Al2O3 can be dissolved in MgSiO3 and that at pressures above 27 GPa MgSiO3 perovskite and Al2O3 corundum form coexisting solid solutions. No further aluminous phase has been observed up to the pressure of the Al2O3 phase transformation to the Rh2O3(II) structure at 80-100 GPa. To what extent the recently discovered high pressure phases of MgSiO3 and Al2O3 will change reciprocal solubilities of the phases in the MgSiO3-Al2O3 system is still unknown. Using static ab initio point defect calculations and simple thermodynamic models, qualitatively correct solid solution behavior of the MgSiO3-Al2O3 system was predicted. The solubility of Al in MgSiO3 is large throughout the mantle and increases with pressure and temperature. Even though the high pressure phase transitions reduce the reciprocal solubilities, these are always large enough to completely assimilate the Al of the pyrolitic mantle. Information on the solubility of Al in MgSiO3 might elucidate mineralogically more complex systems in the lower mantle of the Earth. Incorporation of other impurities present in significant quantities in the Earths mantle (Fe,Fe, and to a lesser extent Cr), into the MgSiO3 host might influence the Al-solubility, and thus change the now well established behaviour in the MgO-AlO-SiO system.